On Jan. 30, 1962, three schoolgirls started giggling in a boarding school classroom in the northeastern corner of what is now Tanzania—and touched off a very strange epidemic. The three couldn’t stop laughing—and soon the uncontrollable cackles spread to their classmates. The laughing attacks lasted from a few minutes up to a few hours; one poor girl reportedly experienced symptoms for 16 straight days. Victims couldn’t focus on their schoolwork, and would lash out if others tried to restrain them.

When 95 of the school’s 159 pupils had come down with what came to be known as omuneepo, the Swahili word for laughing disease, the school shut down. The students returned to their villages, taking omuneepo with them. The affliction spread from person to person, school to school, village to village. “The education of the children is being seriously interfered with and there is considerable fear among the village communities,” noted local medical officers in a 1963 report in the Central African Journal of Medicine. They could find no explanation for the matter. When the epidemic finally died down months later, roughly a thousand people had been struck by the “laughing disease.”

As part of our effort to understand what makes people laugh, we traveled to northeastern Tanzania, tracing omuneepo’s spread across the region more than a half-century ago. We tracked down teachers, students, and medical experts who experienced the phenomenon firsthand. We learned there was nothing funny at all about the situation at the time. The religious boarding school where the laughter began was marked by strict rules, windowless dorms, and devilishly uncomfortably chairs designed to promote correct posture. Investigators found similar conditions at other locations where the omuneepo later erupted: Serious overcrowding, poor food quality.

“It’s a form of complaint,” Kroeber Rugliyama, a longtime local psychiatrist said of the mysterious laughter. “They had no alternative form of expression.”

Laughter is a vexing subject even when it’s not spreading through the countryside like a virulent disease. Take the work of Robert Provine, a neuroscientist and psychology professor at the University of Maryland, Baltimore County. For his book, Laughter: A Scientific Investigation, Provine engaged in what he called “sidewalk neuroscience,” tracking and observing real-world laughter. He and his collaborators used tape recorders to capture more than a thousand “laugh episodes” in bars, shopping malls, cocktail parties, and class reunions. And he had dozens of student volunteers note in a “laugh log” the circumstances around every time they tittered, chuckled, or guffawed.

The results were surprising, even to Provine: Less than 20 percent of the real-world laughter incidents he cataloged were in response to anything resembling something funny. Far more often, people were giggling or chuckling at innocuous statements such as “I’ll see you guys later,” “I see your point,” and “Look, it’s Andre!” What’s more, in all of these cases, the person who produced the laugh-provoking statement was 46 percent more likely to be the one chuckling than the person listening. And while laughter might seem like something that can erupt at any point in response to something funny, in only eight of the 1,200 laugh episodes Provine cataloged did the laughter interrupt what somebody was saying. Instead, 99.9 percent of the time, laughter occurred in tidy, natural breaks in the conversation, punctuating the speech like a period or exclamation point.

Provine discovered that the laughter of our everyday lives isn’t for the most part in response to anything resembling jokes. Instead, most of it occurs in conversations that, out of context, don’t seem funny at all. Provine’s discoveries suggest that laughter is inherently social, that at its core it’s a form of communication and not just a byproduct of finding something funny. Sure enough, when Provine went through the laugh logs he’d collected, he found his participants were 30 times more likely to laugh in the presence of others than when they were alone. Among the few solitary instances of laughter, nearly all occurred in response to TV shows or other media—that is, electronic proxies for other people. When people noted in their journals that they were truly alone, they hardly recorded any laughter at all.

So why would we have evolved the odd and powerful vocal mannerism of laughter? Why do we have an innate need to share what we find funny with others, and why can it can resemble an out-of-control disease?

Evolutionary theory is rife with possible explanations, but one of the most compelling was put forward in a 2005 Quarterly Review of Biology article by an undergrad named Matthew Gervais and his adviser, evolutionary biologist David Sloan Wilson. It’s based on the efforts of a quirky 19th-century French physician named Guillaume Duchenne, who went around zapping people’s faces with electrodes. Luckily for Duchenne, he worked at an old woman’s hospice, so he had access to a lot of prone bodies. He must have been quite the charmer. According to articles on Duchenne, all the ladies wanted to be electrocuted by the “little old man with his mischief box.”

Applying the prongs of his box to people’s faces, Duchenne evoked one kind of smiling—the voluntary kind, the type of expression we produce when we a grin to be polite. This mannerism, he discovered, involves the face’s zygomatic major muscles raising the corners of the mouth. But Duchenne discovered there was a second variety of smiling and laughing, one that occurs when we find something truly entertaining or funny. This expression was more complex, utilizing both the zygomatic major muscles and the orbicularis oculi muscles that form crow’s feet around your eyes. It’s why people say a real smile is in the eyes. Duchenne was never able to reproduce with his electrodes this second form of expression—now known as a Duchenne smile or Duchenne laughter—and he came to believe it was “only put at play by the sweet emotion of the soul.”

More than a century later, Gervais and Wilson saw Duchenne’s discovery as evidence that laughter evolved at two different points in human development. First, they posited, at a point sometime between 2 million and 4 million years ago, came Duchenne laughter, the kind triggered by something funny. An outgrowth of the breathy panting emitted by primates during play fighting, it likely appeared before the emergence of language. This sort of laughter was a signal that things at the moment were OK, that danger was low and basic needs were met, and now was as good a time as any to explore, to play, to socialize. “What the humor is indexing and the laughter is signaling is, ‘this is an opportunity for learning,’” Gervais told us. “It signals this is a non-serious novelty, and recruits others to play and explore cognitively, emotionally and socially with the implications of this novelty.”

But then, sometime in the hundreds of thousands of years after that, theorized Gervais and Wilson, the other sort of laughter emerged—the non-Duchenne sort, the kind that isn’t dependent on something being funny. As people developed cognitively and behaviorally, they learned to mimic the spontaneous behavior of laughter to take advantage of its effects. They couldn’t get it right—they couldn’t simulate the eye-muscle movements of real laughter and smiling—but it was close. Mimicked laughter was a way to manipulate others—sometimes for mutually beneficial purposes, sometimes for more devious reasons. As Gervais and Wilson put it in their paper, “non-Duchenne laughter came to occur in aggressive, nervous, or hierarchical contexts, functioning to signal, to appease, to manipulate, to deride, or to subvert.”

Laughter, in other words, is more than just a response to humor. It’s a primal human tool, one of the building blocks of society. It taps into the core of what we are as social creatures, expressing from one person to another what often cannot be said in any other way: either that everything is in good fun—or, as in the case of omuneepo, that something is very, very wrong.